The present invention relates to semiconductor processing systems and, more particularly, to using focused ion beam deposition in the fabrication of semiconductor devices.
Semiconductor devices employ layers and/or areas of differently-doped semiconductor materials. Dopants are impurities added to the material to change its electrical properties. Dopants generally provide donor ions that "donate" electrical carriers to the material. The carriers can be negatively charged electrons or positively charged holes. The dopant or donor ions are distributed within the material through which the carriers travel; the carriers scatter off donors within their travel direction, decreasing their mobility. Traditional devices built from doped semiconductor materials are usually limited in their speed of operation by the reduced carrier mobility.
Modulation doping is a relatively recent semiconductor technology intended to improve carrier mobility. Modulation doping physically separates in the device the donor ions from the carrier gas induced in the semiconductor materials. The carriers experience far less impurity scattering and their mobility is increased by as much as four to five orders of magnitude. Devices formed from modulation doped materials (such as field-effect transistors) operate far faster than conventional devices.
Current techniques for modulation doping introduce dopant atoms during growth in that part of the structure that will separate them from the carrier gas they induce. Normally the structure is grown using molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). Typically, one layer (for example, of AlGaAs) holds the donor atoms, while the two-dimensional electron gas (2DEG) is induced in an adjacent inversion layer (for example, of GaAs). Modulation doping has been applied for both electron and hole two-dimensional gases. In this case, the inversion layer created at the AlGaAs-GaAs interface will collect.
Unfortunately, current modulation doping techniques are not useful for fabricating complex, heterogeneous integrated devices. The nature of the MBE and MOCVD processes for growing the modulation doped layers render semiconductor wafers uniformly doped over their entire surface. Current technological and commercial uses require more sophisticated integrated circuits that would require differentially doped areas, either in size, shape or in relative concentration and nature of doping (p or n type). Conventional photolithographic techniques (such as masking off non-doped areas) are of little use, since both MBE and MOCVD require ultra-clean surfaces: any contamination from a masking step would ruin the final device.
The present methods for forming modulation doped structures do not provide a complete and flexible system for semiconductor fabrication. What is needed is an improved method for adding dopant ions selectively across a semiconductor surface. An improved modulation doping method should allow precise location of donor ions substantially away from active electrical regions where the highly mobile two-dimensional carrier gases are formed. The improved method ideally would provide techniques applicable to a wide range of semiconductor materials, electrical devices and fabrication geometries. The method should provide a simple and cost-effective technology for accurately adding dopant ions to any semiconductor substrate, for any purpose.